diff --git a/pyproject.toml b/pyproject.toml
index a2e5fb9..bbd4baf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -65,6 +65,12 @@ statmatch = [
 l0 = [
     "l0-python>=0.4",  # L0 regularization for HardConcreteCalibrator
 ]
+calibrate = [
+    # Identity-preserving gradient-descent chi-squared adapter.
+    # microcalibrate pins Python >=3.13; the marker avoids breaking
+    # microplex installs on 3.11/3.12 that don't need the extra.
+    "microcalibrate>=0.22; python_version >= '3.13'",
+]
 benchmark = [
     "sdv>=1.0",  # CT-GAN, TVAE, Copulas
     "scikit-learn>=1.3",  # For QRF comparison
@@ -78,7 +84,7 @@ docs = [
     "sphinx-autodoc-typehints>=1.23",
 ]
 all = [
-    "microplex[dev,benchmark,docs]",
+    "microplex[dev,benchmark,docs,calibrate]",
 ]
 
 [project.urls]
diff --git a/src/microplex/calibration.py b/src/microplex/calibration/__init__.py
similarity index 98%
rename from src/microplex/calibration.py
rename to src/microplex/calibration/__init__.py
index 13ad20b..c5ebc3a 100644
--- a/src/microplex/calibration.py
+++ b/src/microplex/calibration/__init__.py
@@ -2009,3 +2009,22 @@ def validate(self, data: pd.DataFrame) -> dict[str, Any]:
         results["rmse"] = self.calibration_error_
 
         return results
+
+
+
+# --- Optional microcalibrate adapter ---------------------------------------
+# Imported when `microplex[calibrate]` extra is installed. The adapter
+# pulls in torch/optuna/l0 (~1.5 GB) which microplex does not otherwise
+# require, so we keep it opt-in. Callers that need identity-preserving
+# gradient-descent chi-squared calibration install the extra and import
+# `MicrocalibrateAdapter` / `MicrocalibrateAdapterConfig` from this
+# namespace.
+try:
+    from microplex.calibration.microcalibrate_adapter import (  # noqa: F401
+        MicrocalibrateAdapter,
+        MicrocalibrateAdapterConfig,
+    )
+except ImportError:  # microcalibrate not installed
+    MicrocalibrateAdapter = None  # type: ignore[assignment]
+    MicrocalibrateAdapterConfig = None  # type: ignore[assignment]
+
diff --git a/src/microplex/calibration/microcalibrate_adapter.py b/src/microplex/calibration/microcalibrate_adapter.py
new file mode 100644
index 0000000..a13f149
--- /dev/null
+++ b/src/microplex/calibration/microcalibrate_adapter.py
@@ -0,0 +1,224 @@
+"""Country-agnostic adapter that wraps `microcalibrate.Calibration`.
+
+Presents the legacy `microplex.calibration.Calibrator.fit_transform`
+surface on top of a gradient-descent chi-squared solver so country
+packages (`microplex-us`, `microplex-uk`, etc.) share one
+identity-preserving calibrator instead of duplicating the glue. Every
+input record survives to the output with a non-negative weight.
+
+`microcalibrate` is an optional upstream dependency installed via the
+``microplex[calibrate]`` extra. This module raises `ImportError` at
+top-level if the extra isn't installed; `microplex.calibration`'s own
+``__init__.py`` imports from here inside a ``try/except`` so callers
+get the adapter when the extra is present and a clean no-op otherwise.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any, Sequence
+
+import numpy as np
+import pandas as pd
+from microcalibrate import Calibration
+
+from microplex.calibration import LinearConstraint
+
+
+@dataclass(frozen=True)
+class MicrocalibrateAdapterConfig:
+    """Hyperparameters for `MicrocalibrateAdapter`.
+
+    Defaults mirror `microcalibrate.Calibration`'s own defaults
+    (epochs=32, learning_rate=1e-3, noise_level=10.0) except ``device``,
+    which microcalibrate auto-selects CUDA > MPS > CPU but we leave as
+    None so callers keep deterministic control.
+    """
+
+    epochs: int = 32
+    learning_rate: float = 1e-3
+    noise_level: float = 10.0
+    dropout_rate: float = 0.0
+    device: str | None = None  # None = let microcalibrate auto-select
+    seed: int = 42
+    regularize_with_l0: bool = False
+    l0_lambda: float = 5e-6
+    init_mean: float = 0.999
+    temperature: float = 0.5
+    sparse_learning_rate: float = 0.2
+    # Keep activation memory bounded at country-scale pipelines. 100k
+    # records per backward step keeps per-batch autograd activation
+    # under ~200 MB at k = 500 constraints (100_000 * 500 * 4 B).
+    # None = full-batch, which can OOM past ~500k records.
+    batch_size: int | None = 100_000
+
+
+class MicrocalibrateAdapter:
+    """Drop-in replacement for `Calibrator.fit_transform` / `validate`.
+
+    Usage:
+
+        >>> adapter = MicrocalibrateAdapter()
+        >>> result = adapter.fit_transform(
+        ...     data=households_df,
+        ...     weight_col="household_weight",
+        ...     linear_constraints=tuple_of_LinearConstraints,
+        ... )
+        >>> validation = adapter.validate(result)
+
+    The returned DataFrame is a copy of ``data`` with ``weight_col``
+    updated.
+    """
+
+    def __init__(
+        self,
+        config: MicrocalibrateAdapterConfig | None = None,
+    ) -> None:
+        self.config = config or MicrocalibrateAdapterConfig()
+        self._last_calibration: Calibration | None = None
+        self._last_constraint_names: list[str] | None = None
+        self._last_targets: np.ndarray | None = None
+        self._last_performance: pd.DataFrame | None = None
+
+    def fit_transform(
+        self,
+        data: pd.DataFrame,
+        marginal_targets: dict[str, dict[str, float]] | None = None,
+        continuous_targets: dict[str, float] | None = None,
+        *,
+        weight_col: str = "weight",
+        linear_constraints: Sequence[LinearConstraint] = (),
+    ) -> pd.DataFrame:
+        """Calibrate weights via gradient-descent chi-squared.
+
+        ``marginal_targets`` and ``continuous_targets`` are accepted for
+        signature parity with the legacy `Calibrator`, but this adapter
+        expects constraints to be expressed as `LinearConstraint` rows.
+        Callers should compile their marginal / continuous targets into
+        linear constraints before calling.
+        """
+        if weight_col not in data.columns:
+            raise ValueError(
+                f"MicrocalibrateAdapter: weight column {weight_col!r} "
+                f"not found in data (columns: {list(data.columns)[:10]}...)"
+            )
+
+        n_records = len(data)
+        initial_weights = data[weight_col].to_numpy(dtype=float)
+
+        if not linear_constraints:
+            # Nothing to calibrate — preserve caller expectations.
+            self._last_calibration = None
+            self._last_constraint_names = []
+            self._last_targets = np.empty(0, dtype=float)
+            self._last_performance = None
+            return data.copy()
+
+        target_names = [c.name for c in linear_constraints]
+        targets = np.array([c.target for c in linear_constraints], dtype=float)
+
+        for constraint in linear_constraints:
+            if constraint.coefficients.shape != (n_records,):
+                raise ValueError(
+                    f"MicrocalibrateAdapter: constraint {constraint.name!r} has "
+                    f"coefficients shape {constraint.coefficients.shape}, expected "
+                    f"({n_records},) matching the data length."
+                )
+
+        # float32 keeps the adapter's peak allocation at half the
+        # float64 default; microcalibrate casts to float32 anyway, so
+        # this is a free precision-compatible win.
+        estimate_matrix = pd.DataFrame(
+            {
+                c.name: np.asarray(c.coefficients, dtype=np.float32)
+                for c in linear_constraints
+            }
+        )
+
+        calibrator = Calibration(
+            weights=initial_weights,
+            targets=targets,
+            target_names=np.array(target_names),
+            estimate_matrix=estimate_matrix,
+            epochs=self.config.epochs,
+            learning_rate=self.config.learning_rate,
+            noise_level=self.config.noise_level,
+            dropout_rate=self.config.dropout_rate,
+            device=self.config.device,
+            seed=self.config.seed,
+            regularize_with_l0=self.config.regularize_with_l0,
+            l0_lambda=self.config.l0_lambda,
+            init_mean=self.config.init_mean,
+            temperature=self.config.temperature,
+            sparse_learning_rate=self.config.sparse_learning_rate,
+            batch_size=self.config.batch_size,
+        )
+
+        performance_df = calibrator.calibrate()
+        self._last_calibration = calibrator
+        self._last_constraint_names = target_names
+        self._last_targets = targets
+        self._last_performance = performance_df
+
+        result = data.copy()
+        result[weight_col] = calibrator.weights
+        return result
+
+    def validate(self, calibrated: pd.DataFrame | None = None) -> dict[str, Any]:
+        """Return validation metrics in the shape the legacy pipeline expects.
+
+        The legacy `Calibrator.validate` returns ``{"converged",
+        "max_error", "sparsity", "linear_errors"}``. We populate the
+        same keys. ``calibrated`` is accepted for interface parity but
+        not read; the authoritative values come from the last
+        ``calibrate()`` call.
+        """
+        if self._last_calibration is None:
+            return {
+                "converged": True,
+                "max_error": 0.0,
+                "sparsity": 0.0,
+                "linear_errors": {},
+            }
+
+        estimates = self._last_calibration.estimate().to_numpy(dtype=float)
+        targets = self._last_targets
+        names = self._last_constraint_names
+
+        rel_errors = np.where(
+            np.abs(targets) > 1e-12,
+            np.abs(estimates - targets) / np.abs(targets),
+            np.abs(estimates - targets),
+        )
+        linear_errors = {
+            name: {
+                "target": float(target_value),
+                "estimate": float(estimate_value),
+                "relative_error": float(rel_error),
+                "absolute_error": float(abs(estimate_value - target_value)),
+            }
+            for name, target_value, estimate_value, rel_error in zip(
+                names, targets, estimates, rel_errors, strict=True
+            )
+        }
+
+        max_error = float(rel_errors.max()) if rel_errors.size else 0.0
+        weights = self._last_calibration.weights
+        sparsity = float((weights == 0).sum()) / max(len(weights), 1)
+
+        return {
+            "converged": bool(max_error < 0.05),  # 5 % relative error bar
+            "max_error": max_error,
+            "sparsity": sparsity,
+            "linear_errors": linear_errors,
+        }
+
+    def performance_history(self) -> pd.DataFrame | None:
+        """Per-epoch performance log from microcalibrate, if available."""
+        return self._last_performance
+
+
+__all__ = [
+    "MicrocalibrateAdapter",
+    "MicrocalibrateAdapterConfig",
+]
diff --git a/tests/test_microcalibrate_adapter.py b/tests/test_microcalibrate_adapter.py
new file mode 100644
index 0000000..37bd735
--- /dev/null
+++ b/tests/test_microcalibrate_adapter.py
@@ -0,0 +1,87 @@
+"""Upstream-home tests for the country-agnostic microcalibrate adapter.
+
+The adapter wraps `microcalibrate.Calibration` with the legacy
+`Calibrator.fit_transform` interface so country packages (microplex-us,
+microplex-uk, etc.) share one identity-preserving calibrator instead of
+duplicating the glue code.
+
+microcalibrate is an optional extra; these tests import it via the
+`microplex[calibrate]` route and skip gracefully when unavailable.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+import pandas as pd
+import pytest
+
+from microplex.calibration import LinearConstraint
+
+microcalibrate = pytest.importorskip("microcalibrate")
+
+
+def _toy_data(n_records: int = 200, seed: int = 0) -> pd.DataFrame:
+    rng = np.random.default_rng(seed)
+    return pd.DataFrame(
+        {
+            "age": rng.integers(18, 70, size=n_records),
+            "weight": np.ones(n_records),
+        }
+    )
+
+
+def _age_band(
+    data: pd.DataFrame, name: str, low: int, high: int, target: float
+) -> LinearConstraint:
+    mask = (data["age"] >= low) & (data["age"] < high)
+    return LinearConstraint(
+        name=name,
+        coefficients=mask.astype(float).to_numpy(),
+        target=target,
+    )
+
+
+class TestImportSurface:
+    """The adapter must be importable from the canonical upstream location."""
+
+    def test_importable_from_microplex_calibration(self) -> None:
+        from microplex.calibration import (
+            MicrocalibrateAdapter,
+            MicrocalibrateAdapterConfig,
+        )
+
+        assert MicrocalibrateAdapter is not None
+        assert MicrocalibrateAdapterConfig is not None
+
+    def test_default_config_sets_batch_size(self) -> None:
+        """Default batch_size is set so v7-scale fits stay under RSS budget."""
+        from microplex.calibration import MicrocalibrateAdapterConfig
+
+        config = MicrocalibrateAdapterConfig()
+        assert config.batch_size is not None
+        assert config.batch_size > 0
+
+
+class TestConvergesOnSmallProblem:
+    """End-to-end check: fit_transform moves weights toward targets."""
+
+    def test_three_age_bands_converge(self) -> None:
+        from microplex.calibration import (
+            MicrocalibrateAdapter,
+            MicrocalibrateAdapterConfig,
+        )
+
+        data = _toy_data(n_records=300)
+        constraints = (
+            _age_band(data, "age_18_30", 18, 30, 60.0),
+            _age_band(data, "age_30_45", 30, 45, 90.0),
+            _age_band(data, "age_45_70", 45, 70, 150.0),
+        )
+        adapter = MicrocalibrateAdapter(
+            MicrocalibrateAdapterConfig(
+                epochs=400, learning_rate=0.05, noise_level=0.0
+            )
+        )
+        result = adapter.fit_transform(data, linear_constraints=constraints)
+        validation = adapter.validate(result)
+        assert validation["max_error"] < 0.1, validation